To permanently add a charm:
With the Xbox version, at least in D2:ED, just select a piece of equipment with one or more empty charm slots in the inventory and hit A; a menu will come up allowing you to permanently charm the item with any of the charms you have collected. In the equipment menu you can hit Y to add charms to equipped items (the only way to charm a bow in D2:ED).
On the PC right click a piece of equipment with an empty charm slot.
The content of a merchant's inventory, mineral veins, unlocked chests, etc are set when you get close enough and run the crosshairs over them. If you save before doing so you can reload repeatedly to try to get something good (with a merchant, their inventory will be reset the next time you level). I wouldn't waste too much time with this, though, unless you really need to upgrade something, or are at a chest (or quest reward, boss type opponent, etc) that always gives something decent quality (and you want to make sure you get something that suits your character) or at a malachite vein to make sure you get some gems (1 or 2 are relatively easy to get, 3 can be gotten with a bit of patience or luck, and 4 are possible but quite rare).
